The present invention relates to a method for depositing a carbon film on a substrate and products obtained thereby.
Moderately energetic ions or neutral atoms of carbon can be used to form a hard film on a substrate. The carbon film is more similar to diamond in properties than to graphite and therefore has been referred to as I (ion) - carbon or diamond-like carbon. The structure of diamond-like carbon is not precisely understood but it is known to be harder than hardened steel though not as hard as diamond. It is also known to be a good insulator, chemically inert, and has a refractive index similar to diamond. However, the specific properties of diamond-like carbon depend on the deposition conditions such as ion energy, substrate temperature and the amount and type of other ions which may impinge on the film.
One application of such films is as hard, protective coatings for optical lenses. Another application is in the medical field for devices which are inserted into the human body. Because the carbon film produced by the process of this invention is chemically inert and an insulator, such coated medical devices possess anti-clotting properties (i.e. do not substantially cause the formation of thrombi). Still further, the present process may be adapted to provide for the homoepitaxial growth of diamonds [See, Nature vol. 275 pp. 634-635 (Oct. 19, 1978)].
Several techniques have been used to develop diamond-like carbon films. For example, J. Appl. Phys. 42, 2953 (1971) discloses the deposition of diamond-like carbon using a carbon-argon beam extracted from an ion source in which an argon glow discharge takes place between an anode and a graphite cathode. This cathode is sputtered by the discharge and some sputtered carbon atoms are ionized in the discharge. A disadvantage of this technique is that the concentration of carbon ions in the discharge is very low. Hence another preferred technique is to crack a hydrocarbon gas in either a DC or radiofrequency (RF) glow discharge. The arrangement in the latter case is analogous to one used in RF sputtering devices. The substrate is placed on an electrode connected to the center conductor of a coaxial cable through a matching network. The substrate assumes a negative bias, the amount of which depends on the power input.
The foregoing techniques produce films which often contain undesirable amounts of graphite which reduces resistance and light transmission and, in large enough concentrations, have an adverse effect on hardness. It appears that as the thickness of the film increases, the proportion of graphite increases because of the formation of graphite nucleation centers.
Additionally, many prior art diamond-like carbon films contain large amounts of hydrogen (about 30 atomic %). This results in compressive mechanical stress and infrared absorption by the film due to C--H bonds. As a result, the thickness of prior art films has not exceeded about one micron.
The prior art techniques as cited above teach that the amount of chemically active gases other than hydrocarbon gases (e.g. O.sub.2) should be severely limited to avoid removal of the coating. In contradistinction, applicants have found that ions of these gases preferentially remove undesirable graphite from the substrate film resulting in an improved diamond-like carbon coating.
Since some of the diamond-like carbon is also removed by the graphite removing gases, the amount of these gases must not be excessive. It was generally assumed that these gases must be avoided and must be present in trace amounts, if at all. Contrary to this teaching, Applicants found that these gases improve the coating and can be present in larger amounts without unduly reducing the rate of deposition.
It is therefore an object of the present invention to provide a method of forming a diamond-like carbon film on a substrate which has reduced amounts of graphite and hydrogen. It is a further object of the present invention to provide a diamond-like carbon film which can have thickness exceeding one micron.